Grain-oriented electrical steel sheets having an axis of easy magnetization in a rolling direction of a steel sheet are used as an iron core of a power converter such as a transformer. Low core loss characteristics are strongly demanded for an iron core material so as to reduce a loss caused by energy conversion.
As an example of methods for reducing an core loss, there has been proposed a method for reducing an eddy current loss that largely accounts for the core loss by imparting a stress to the surface of a steel sheet or providing a linear groove therein, and thereby subdividing a 180-degree magnetic domain.
However, when the method of imparting the stress to the steel sheet surface is employed, the stress is relieved by heat treatment in a case in which stress-relief annealing is required in assembling a transformer such as a wound iron core. As a result, the eddy current loss reduction effect by subdividing the magnetic domain disappears.
Meanwhile, when the linear groove is physically fabricated in the steel sheet surface, the eddy current loss reduction effect by subdividing the magnetic domain remains even after the stress-relief annealing.
A plurality of methods have been proposed as the method for fabricating the groove in the steel sheet surface, and examples thereof are disclosed in Patent Literatures 1 to 5. However, the techniques disclosed in Patent Literatures 1 to 5 relate to a method for fabricating a simple and continuous linear groove.
Meanwhile, when a groove composed of a main linear groove (referred to as main groove below) and a plurality of sub line-segmented micro grooves (referred to as sub-groove below) branching therefrom is fabricated in the steel sheet surface, more excellent core loss characteristics are obtained as compared to the case in which the simple linear groove is fabricated.
However, the branching grooves as described above cannot be fabricated by directly using the fabrication methods disclosed in Patent Literatures 1 to 5.
That is, when etching is performed to fabricate the branching micro grooves in the steel sheet surface to a depth at which desired core loss characteristics are obtained, an interval between the branching micro grooves becomes smaller. As a result, there occurs a problem that the micro grooves adjacent to each other become continuous to each other, to thereby form a wider main groove.